TOKYO One day before he tried to hand a letter to a top Japanese lawmaker offering to kill hundreds of disabled people, the suspect in Japan's worst mass killing in decades tweeted: "I don't know if it's right, but action is the only way."

Less than six months later, Satoshi Uematsu, 26, was arrested on suspicion of stabbing 19 people to death and wounding dozens of others as they slept at a centre for the disabled where he had worked for more than three years until February.

Police have yet to comment on the motive for Tuesday's killing spree, which has shocked a nation with one of the lowest crime rates in the world and where mass murders are rare.

Information about Uematsu is still emerging, but interviews with neighbours and posts on his Twitter account paint a picture of an outwardly polite young man who became obsessed with the people being cared for at the Tsukui Yamayuri-En facility for the mentally and physically disabled in Sagamihara town, about 25 miles (40 km) southwest of Tokyo.

Akihiro Hasegawa, who lived next door for eight years, said he'd noticed nothing strange about Uematsu, a college graduate.

"He always smiled when he greeted me. He was very mannerly and polite. A really nice young man," Hasegawa, 73, told Reuters.

"It would be easier to understand if there'd been a warning but there were no signs," said Hasegawa, adding he thought Uematsu's experiences on the job might have affected his mind.

"We didn't know the darkness of his heart."

A Twitter account carrying Uematsu's name and which domestic media said was used by the suspect indicated he was a fun-loving young man who enjoyed karaoke and beach parties and wanted to quit smoking.

"I'm super happy," he tweeted on June 29 with photos at what appeared to be a friend's wedding party.

He was also a fan of a conspiracy theory card game, and posted that he saw hidden messages such as a prediction of Japan's March 2011 nuclear disaster in the cards.

SUDDEN CHANGE

Uematsu had come to authorities' attention in February, when he said he could "obliterate" 470 disabled people.

"I am fully aware this is a statement that defies common sense," he said in letters addressed to the speaker of parliament's lower house, a copy of which was obtained by Kyodo news agency.

"However, I could not stand idle as I thought about the exhausted look on the faces of their caretakers, the crazed look in the eyes of the staff working at the facilities, and, in the best interest of Japan and the world, I have been moved to take this action today."

He added: "My goal is a world in which the severely disabled can be euthanized, with their guardians’ consent, if they are unable to live at home and be active in society."

Yuji Kuroiwa, the governor of Kanagawa prefecture where the facility is located, said he had been told that Uematsu had suddenly changed before he was encouraged to voluntarily resign in February.

"Nothing much happened during the five months" after he left his job, Kuroiwa added. "You could say there were warning signs, but it’s difficult to say if this could have been prevented."

Uematsu promised in the letters to execute the killings swiftly, without hurting staff and said he hoped to be found "not guilty by reason of insanity".

Uematsu also told local police that he would kill many severely disabled people if the government approved the action and on Feb. 19, he was involuntarily committed to a mental hospital for fear he would harm others, a Sagamihara city official said. Tests showed he had used marijuana and suffered from paranoia, NHK public TV reported.

"Voluntary resignation from my company. I just might get arrested," he tweeted that same day.

Twelve days later, on March 2, doctors judged his condition had improved enough to discharge him.

After the July 22 killing of nine people in Munich, Germany, Uematsu tweeted a photo of two Japanese men holding what appear to be toy guns, with the message "No gun, Yes toy" and "Shooting in Germany at the same time - it's fun if it's toys".

Around the time of the stabbings on Tuesday, the Twitter account showed a close-up photo of a young man wearing a red-tie, white collar shirt and jacket.